MAMMALS PECULIAR TO ALASKA 93 



north-eastern Asia. Professor Engler* cites a large number 

 of arctic-alpine plants, most of which are met with in these 

 two regions, and extend southward in America along the 

 Eocky Mountain chain. He argues that some of the species 

 were probably capable of spreading from the one continent 

 to the other under the existing conditions of land and water. 

 Of others he does not consider that they could have drifted 

 across the ocean by occasional means of transport, and he 

 urges that only a former land connection in the north could 

 satisfactorily explain their presence in Asia and North 

 America. 



Finally a few observations on the land bridge problem from 

 the marine faunal aspect may elucidate some doubtful points. 

 Dr. Dallf divides the existing marine fauna of the north- 

 eastern Pacific into three zones, viz., the Oregonian, Aleutian 

 and Arctic. The Oregonian extends from Monterey to the 

 Shumagin Islands. The Aleutian passes westward from the 

 Shumagin Islands, includes the Aleutian chain, and reaches 

 northward to the floating ice in Bering Sea, bounded by a 

 depth of five hundred fathoms. The arctic zone stretches 

 indefinitely southward beyond the five-hundred-fathom line, 

 and is bounded in the north by floating ice, or water of thirty- 

 two degrees. Some arctic species have crept southward to the 

 northern islands of Japan and Cape Newenham, while a series 

 of isolated arctic colonies occur from Cook's Inlet southward 

 and eastward. 



Dr. Dall's views are mostly derived from a study of the 

 marine mollusca, and it is interesting to note that in the 

 main points they are confirmed by Miss Eathbun J after an 

 examination of the northern Pacific Crustacea. She states 

 that arctic species often continue southward through Bering 

 Strait, along the west coast of Bering Sea, to Okhotsk Sea and 

 the Kurile Islands. Some of them extend eastward to Puget 

 Sound and even further south. Miss Eathbun noticed that in 

 exceptional cases, for instance Philyra pisum and Cancer 



* Engler, A., " Entwicklungsgeschichte der nordl. Florengebiete," 

 pp. 2243. 



t Ball, W. H., " Marine Faunal Eegion of North Pacific," p. 206. 



\ Eathbun, Mary J., " Decapod Crustaceans of North- West Coast," 

 p. 6. 



